I am sorry you are going through this so often with your boyfriend. My experience has been that this type of response, pulling away because of your partner’s past, is a cover for something else.
Offer to go to couples therapy with him. I highly recommend working together to sort this through. I believe that this is your best effort in keeping the relationship. It can help get to the bottom of it.
But, if he is unwilling to go with you I wouldn’t waste much time trying to convince him. If he can’t see the good person that’s in front of him right now, he can always use the past to break it off — and you deserve better than that.